Investigation of carbon in the Allan Hills A77278 meteorite via 2D and 3D Raman microspectroscopy

This paper reports the first Raman spectral and imaging data on the ordinary chondrite Allan Hills A77278 (ALHA77278) in two and three dimensions. Mineralogy in the ALHA77278 meteorite appears to be mainly silicates (olivine and pyroxene) with some carbonaceous material in the matrix. Metal phases are also present in the studied sample. Two-dimensional Raman spectral distribution maps show the locations of these chemical constituents. Several diamond grains are observed to be buried in olivine and extend down to a similar to 7-mu m depth in the sample, which might be contamination captured during the sample preparation. Raman spectral peak parameters for D and G carbon bands are presented and trends among these parameters are discussed. These spectral parameters, when compared with those of other meteorites reported in the literature, show similarity with unequilibrated ordinary chondrites and primitive carbonaceous chondrites. Estimated peak metamorphic temperature for the parent asteroid is calculated to be around similar to 260 degrees C, which is comparable with lower metamorphic grade ordinary chondrites, indicating that the metamorphic grade of ALHA77278 might be less than 3.7, as it has been officially assigned. Three-dimensional Raman tomographic measurements present visualizations of the identified chemical components in high resolution as well as the epoxy used during the preparation of the sample. Spatial distribution of epoxy is used to avoid these regions during the Raman spectral data extraction from the carbonaceous matter from a depth of the sample, which could otherwise complicate the datasets used to infer the thermal history of the ALHA77278 meteorite.